(3aR,7aR)-2,3,3a,4,5,6,7,7a-Octahydro-2-[(R)-alpha-(1-hydroxy-4-tert-butylcyclohexyl)phenyl-methyl]-1,3-dimethyl-1H-1,3,2-benzodiazaphosphole 2-oxide, C25H41N2O2P, M(r) = 432.59, orthorhombic, P2(1)2(1)2(1), a = 10.636 (2), b = 12.759 (1), c = 18.185 (2) angstrom, V = 2467.8 (5) angstrom3, Z = 4, D(x) = 1.164 Mg m-3, lambda(Cu K-alphaBAR) = 1.54178 angstrom, mu = 1.138 mm-1, F(000) = 944, T = 293 K, R = 0.047, wR = 0.039 for 1884) observed reflections. The title compound is constituted of a (1R,2R)-N,N'-dimethyl-1,2-diaminocyclohexane attached to a tetrahedrally coordinated phosphoryl group. A phenylmethyl group with an R configuration bridges the P atom and the 4-tert-butylcyclohexanol ring. The tertiary alcohol is in the axial position and the 4-tert-butyl is in the equatorial position. The molecules are held in the crystal through intermolecular hydrogen bonds of the O-H...O type and by van der Waals interactions.